Spin‐Orbit Coupling of Ln ³⁺ Induces and Modulates Thermally Activated Delayed Fluorescence in Heterometallic LnAgP ₃ Clusters

ABSTRACT The rational design of Ag(I)‐based thermally activated delayed fluorescence (TADF) materials requires fundamental understanding of structure–property relationship governing their emission characteristics. In this work, coordination with Ln 3+ ions endows the resulting compounds LnAgP 3 (Ln = Gd/Eu/Y) with pronounced photoluminescence. Temperature‐dependent emission spectra and decay lifetime measurements reveal that Y 3+ and Gd 3+ incorporation induces distinct TADF activity in the [AgP 3 ] moiety. In EuAgP 3 , combined the experimental results supports efficient energy transfer from [AgP 3 ] to Eu 3+ via both singlet energy transfer (SET) and triplet energy transfer (TET) pathways. Modulating the Eu:Gd molar ratio within a lattice enables precise control over the TADF performance of the [AgP 3 ] unit. At a Eu:Gd ratio of 0.5:0.5, optimal TADF performance of the [AgP 3 ] moiety is observed with a larger k (S 1 →S 0 ) value of 1.05 × 10 7 s −1 and a shorter TADF decay time of 6.45 µs. Theoretical calculations further reveal that the SOC of the 4 f orbitals perturbs the electronic structure of [AgP 3 ], compressing Δ E (S 1 ‐T 1 ) to <0.2 eV, which enables reverse intersystem crossing (RISC) and thus TADF. Consequently, varying the Eu:Gd ratio provides an indirect handle over the SOC‐mediated Ag↔Eu energy‐transfer pathway, offering an effective route to regulate the TADF performance of the [AgP 3 ] moiety.